Giuliano Cappato ? A VLF survey in Vinadio geothermal area
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supposed to be geothermal fields can be analyze indirectly during only one day of
surveying. The most of these methods gives useful information about the shape and the
depth of the geologic structures which represent the geothermic reservoir and
sometimes the heat source that can only be confirmed through exploratory drilling.
The purpose of this thesis is to verify the effectiveness of the VLF (Very Low
Frequencies) geophysical survey method across the geothermal field of Bagni di
Vinadio (Cn), confirming that conductivity contrasts associated with brecciated or
mineralized faults that normally produce VLF anomalies.
VLF surveying is a continuous-wave (frequency domain) electromagnetic technique
that uses low-frequency radio transmissions as the source. When these intersect a buried
conductor, they induce eddy currents that generate a secondary magnetic field
concentric around the source of the currents: VLF surveys involve measuring the
orientation of this field and, as the instrument passes perpendicularly over a vertical
target, the vector orientation changes from a maximum on one side to a minimum on the
The method is primarily used in mineral exploration work but has also been
successfully applied in engineering and groundwater surveys to detect conductive fault
zones, zones of mineralization (which tend to be more conductive than the surrounding
bedrock or host rock) and other sub-vertical conductors.
In particular, in the first chapter of this study it is explained the origin of geothermal
energy, its environmental risks and sustainability. Then, in the second chapter, it is
analyzed the geological usual environment with the most important classifications of
geothermal systems present in literature.
In the third chapter, all the methods that have got a key role in the geothermic
exploration are analyzed, while in the fourth chapter it is studied the site in which the
VLF survey has been carried out. There have been presented the main geographic,
geological and geo-structural aspects which are fundamental in order to explain the
presence of sources of hot water flows in depth.
Then it has been analyzed the active thermal circulation in that zone, focusing on the
geothermal and salinity data and estimating a range of possible formation resistivities
with the Archie method, in order to correlate it with the following VLF survey response.